The invention relates to implantable articles and methods for manufacturing such articles. More particularly, the invention relates to bone prostheses and investment casting processes for manufacturing the same.
There are known to exist many designs for and methods for manufacturing implantable articles, such as bone prostheses. Such bone prostheses include components of artificial joints, such as elbows, hips, knees, and shoulders. An important consideration in the design and manufacture of virtually any implantable bone prosthesis is that the prosthesis have adequate fixation when implanted within the body.
Early designs of implantable articles relied upon the use of cements such as polymethylmethacrylate to anchor the implant. The use of such cements can have some advantages, such as providing an immediate and secure fixation that does not develop free play and lead to erosion of the joining bone faces post-operatively. However, the current trend is to use these cements to a lesser extent because they tend to lose their adhesive properties over time and because they can contribute to wear debris within a replaced joint.
Recently, implantable bone prostheses have been designed such that they encourage the growth of hard tissue (i.e., bone) around the implant. Bone ingrowth usually occurs and growth is promoted where the surface of an implantable bone prosthesis is irregular or textured. The interaction of newly formed hard tissue in and around the textured surface of the implantable bone prosthesis has been established to provide good fixation of the prosthesis within the body. Greater bone fixation can usually be achieved where bone contacting surfaces of an implantable bone prosthesis are more porous or irregular.
Porous or irregular surfaces can be provided in implantable articles by a variety of techniques. In some instances irregular patterns or surface porosity is formed in an implantable bone prosthesis by embossing, chemical etching, milling or machining. One drawback to using such common techniques is the significant amount of post-processing time required. The post-processing operations lead to delays in obtaining the finished product and also significantly increase the cost of manufacturing the device. These post-processing operations can also degrade the mechanical properties of the device.
Textured surfaces are also applied to implantable bone prostheses by joining one or more separate surface plate inserts to an exterior surface of the prosthesis to provide separate porous surfaces or pore-forming surfaces. Separate pore-forming surfaces can be joined to or formed on an implantable bone prosthesis by sintering small metal particles or powders to a surface of the prosthesis in a random pattern. Wire-based pads or grids can also be fused to implantable bone prostheses to provide a texture or surface relief features. A drawback of such techniques is that the components added to form the textured surface can become dislodged from the prosthesis. Dislodgment of these components compromises the fixation mechanics of the implant and can also contribute to wear debris. Further, the sintering step required to fuse texture-forming components to bone prostheses is a high-temperature post-processing step that could impart mechanical weaknesses to the prosthesis, distort the dimensions of the prosthesis, and/or alter the properties of the materials from which the prosthesis is made.
Optimal bone fixation is believed to occur with implants that have more complex and irregular surfaces on a rather small dimensional scale, which provides a larger bone-contacting surface area with some depth of texture. Apparently, hard tissue (i.e., bone) is able to infiltrate small pores and passages that form the textured surface, thus providing firm interlock between the implant and the bone. It is also believed that the best textured surfaces for implantable bone prosthesis are those in which the porous surface is integral with the prosthesis, as opposed to porous surfaces that are separately fused to the prosthesis by post-processing operations.
An ideal textured surface would be one in which the macrotextured, porous region of an as-cast article includes pores with undercut edge profiles. Unfortunately, available technology has not previously enabled the manufacture of implantable articles with such macrotextured, porous surfaces.
Implantable articles such as bone prostheses are often made by an investment casting process. Investment casting first requires the manufacture of a solid model of the article to be cast. The solid model is often made from a meltable casting wax through a molding operation such as injection molding. Once the solid model is made, one or more of the solid models are fixed to a wax tree and the wax tree, together with the attached solid models, is encased in a binder material to form a shell. This is done by repeatedly dipping the assembly in a ceramic slurry coating and drying the coating between dips, to form a shell. After drying, the shell is heated to a temperature sufficient to melt and extract the casting wax from within the shell. Molten metal is then pored into the shell where it fills the cavities once occupied by the solid models, thereby forming cast articles corresponding to the shape of the hollow regions left by the lost wax.
Although it is known to be useful to form implantable bone prostheses having as-cast macroporous textures, it is difficult to do so using the traditional investment casting techniques described above. The preparation of solid models by injection molding techniques is a serious limitation of such a process. It is difficult, if not impossible, to incorporate any suitable macrotextured surface into a solid model to be formed by an injection molding process because release of the model from the mold is likely to destroy the model. If the model has undercut surface features, it cannot be separated from the mold without breaking the model, and/or the mold.
Accordingly, there is a need for bone prostheses having improved textured surface characteristics that enhance the fixation mechanics of the implantable prostheses to hard tissue within the body. There is also a need for improved methods of manufacturing prostheses having such characteristics.
It is thus an object of the invention to provide implantable articles such as bone prostheses having surface characteristics that promote hard tissue ingrowth and improved fixation within the body. It is also an object of the invention to provide implantable bone prostheses having exterior, bone contacting surfaces that include an as-cast, macrotextured region. Another object of the invention is to provide casting techniques that enable the manufacture of implantable bone prostheses having as-cast macrotextured, porous surfaces. A further object of the invention is to provide casting techniques that facilitate the manufacture of implantable bone prostheses with as-cast macrotextured surfaces designed to take advantage of optimum fixation mechanics for a given prosthesis. These and other objects will be apparent from the description that follows.